1. Field of the Invention
This invention relates to a touch sensitive tablet using force detection (hereinbelow, called "touch panel") which serves to select items indicated on a faceplate such as display panel when combined with a display device or to apply patterns to a computer system etc.
2. Description of the Prior Art
There has been known a method wherein the component forces of a touch at three supporting points on a faceplate are converted into electric signals by means of force detectors disposed at the respective supporting points, and the electric signals are subjected to predetermined calculations, thereby to find the coordinates of a touched point on the faceplate. (cf: Japanese Patent Application Publication No. 49-34247)
In case of finding the coordinates of the touched point with this method, a strain gauge, a piezoelectric element or the like is employed as the force detector. After the electric signals corresponding to the component forces at the respective fulcra have been subjected to the necessary analog processings of amplification, shaping etc., the coordinates are evaluated by means of an analog calculator. It is also possible that the electric signals subjected to the necessary analog processings of amplification, shaping etc. are converted into digital signals by means of an analog-to-digital converter (hereinbelow, abbreviated to "ADC"), whereupon the coordinates are evaluated with the digital signals by means of a digital calculator (such as digital computer).
In this case, the calculated values of the coordinates fluctuate due to, not only noise in the force detectors and amplifiers, noise and calculation errors in the calculator, quantization errors in the ADC, etc., but also outputs of the force detectors in the untouched state (offset values).
Heretofore, regarding the fluctuations of the calculated values ascribable to the noise, calculation errors etc. of the various circuits, circuits of low noise and high precision have been used. This has led to the problem that the cost of the apparatus increases. Regarding the fluctuations of the calculated values ascribable to the offset values, since the offset values vary with time due to a temperature rise etc., they have been measured and corrected at suitable intervals. This has resulted in the problem that the coordinate inputs must be interrupted every interval of the measurement of the offset values.